The present invention relates to a novel syntehtic monofilament thread conventionally employed in sewing operations. The synthetic material most commonly employed is nylon and as such, monofilament threads have been utilized commercially for approximately ten years for a variety of sewing operations ranging from the clothing industry to the heavier duty upholstery, furniture and industrial fabrics. Irrespective of the industry and its product, all share in common the joining of two or more layers of fabric with a sewing machine generally including a thread let-off mechanism, a needle and a bobbin mechanism.
One of the reasons nylon monofilament threads are specified rather than the older, standard natural fibers, e.g., cotton, is due to the far superior tensile strength of nylon over natural fibers. In addition to the greater strength initially, nylon tends to have a greater life; being moisture and air resistant, it weathers far better than cotton. The fact that nylon monofilament thread is transparent, substantially eliminating the need for dyeing and keeping large quantities of thread merely to be able to match various fabrics, has further added to its acceptance and desirability over the natural materials.
Notwithstanding the advantages attendant nylon monofilament, the thread is not entirely free from problems and criticism. One of the chief problems encountered is, as the thread is employed with heavier materials, e.g., denim, cotton duck, seat cushions, foam backing, vinyls, and the like, it tends to melt forming a break in the line or a condition known as burn-off. The melting or burn-off is of course due to friction which becomes a problem as thread diameters are increased, particularly beyond 7.0 mils (0.0178 cm) and increasingly more so at diameters of 10.0 mils (0.0254 cm) and greater. For such applications heavier equipment is employed which includes larger needles.
The larger needle becomes quite hot due not only to the friction encountered between its eye and the thread, but also due to the greater resistance it encounters during repeated penetration of the work fabric. The greater tension on the thread also causes friction with resultant heat increases between the thread and the discs. When the heat of friction becomes great enough, one of several things may happen resulting in burn-off or breakage. First, the thread can melt at a point of contact with any of the hot surfaces either during movement of the thread or, more likely, during a pause when it may then stick to the needle. Second, the thread can abrade as it passes over and through the various machine components weakening the thread until it breaks as well as generating heat for a subsequent burn-off. Third, excessive heat can elevate the temperature of the monofilament thread to a point just below its melt at which it will elongate, reducing its cross-sectional area and tensile strength until breakage occurs.
Whenever burn-off or breakage occurs, sewing ceases and rethreading operations commence with an expenditure of time and as much as 20 inches (50.8 cm) of thread in waste. It is not uncommon for a breakage to be experienced with every several inches (cm) of thread sewn making such operations troublesome and unprofitable.
In some instances the thread may not melt through or break, but enough heat is generated for it to stick to the eye of the needle and perhaps the discs. If the thread cannot pass freely through the eye of the needle or its movement is excessively impeded by the discs, skipped stitches can result. When the needle and thread pass through the work fabric a loop should be formed to be united with the bobbin thread. Skipped stitches occur if the thread is sticking to an extent that a loop is not formed at all or is insufficient to be caught by the bobbin thread. At this point, an interruption to free the sticking may allow another hot surface to melt through the thread as discussed previously.
Still another problem encountered with nylon monofilament thread is slipback. Usually, when a seam is started a lock stitch or back stitch is made by running the needle and thread in one spot or backover the first several stitches. However, if the lock or back stitch is not employed because of time and/or appearance considerations, and a seam is sewn in the forward direction, even the least amount of force subsequently applied to the beginning will cause the seam to open due to the inherent stiffness of any synthetic round thread with itself and the fabric through which it passes.
Synthetic threads also tend to be considerably more stiff than the natural fibers and therefore, exposed ends can be sharp to the touch -- an undesirable property when such threads contact the body. Although the exposed ends are seldom injurious to a person, the occasional discomfort caused thereby is annoying.
Finally, from an aesthetic consideration, synthetic round monofilament threads, albeit transparent, reflect light from many angles to a degree that their visibility can detract from the appearance of the fabric or article sewn with such thread. These threads, being round in cross-section and inherently stiff, often do not form close lying seams but rather raised loops above the material not only visible, but also sufficiently exposed to be picked or snagged. Also, for a stitch to be attractive, its optimum formation occurs when the interlocking loop, made by joining the top and bobbin thread, is interposed between the two pieces of fabric and not when the loop of one side is pulled through to the opposite fabric piece. Round synthetic monofilament threads often do not form flat, smooth stitches. This is particularly true when abrasion and heat cause the thread to drag, interferring with the sewing operation. p Although not immediately apparent, one possible reason for all the problems experienced with synthetic monofilament threads is their cylindrical configuration. However, with the exception of the existent multifilament synthetic threads commerically available which are free from some of the problems discussed herein, little success has been made toward satisfactorily improving the sewability of the monofilament threads in spite of considerable activity in this field. Eight U.S. patents which typify various attempts to produce synthetic monofilament thread having different cross-sectional profiles are U.S. Pat. Nos. 1,773,969; 2,816,349; 2,953,839; 3,156,607; 3,298,169; 3,425,893; 3,673,053; and, 3,802,177. While it is not known how satisfactorily these threads may be sewn, several of the patents are directed toward configurations designed to control stiffness and reflectivity. However, none is addressed toward nor discusses the problems of burn-off, skipped stitch, slipback and the like, presented hereinabove.